1. Field of the Invention
The present invention relates to a microchip useful as a μ-TAS (Micro Total Analysis System) used for environmental analysis, chemical synthesis and biochemical inspection of DNA, protein, cells, immunity, blood and the like and, more specifically, to a microchip for inspection and analysis utilizing optical examination.
2. Description of the Background Art
Recently, in the field of medical, health, food and drug discovery, importance of sensing, detecting and determining quantity of chemical substance and biological matter such as DNA (Deoxyribo Nucleic Acid), enzyme, antigen, antibody, protein, viruses and cells has been increasing, and various biochips and micro chemical chips (hereinafter generally referred to as microchips) allowing easy examination of these have been proposed. The microchip enables a series of experiments/analysis operations, which has been conducted in a laboratory, within a chip having the size of a few centimeters to ten centimeters square and the thickness of a few millimeters to a few centimeters. Therefore, it is advantageous in many aspects. For example, it requires small amount of specimen and reagent, its cost is low, reaction speed is fast and hence inspection with high throughput is possible, and the result of inspection can be provided at the site where the specimen is taken.
A microchip has a fluid circuit therein, and the fluid circuit mainly consists of various sections including: a liquid reagent holding section holding liquid reagent for processing a specimen (such as blood) or for causing a reaction with the specimen or to be mixed with the specimen; a measuring section for measuring the specimen or liquid reagent; a mixing section for mixing the specimen with the liquid reagent; a detecting section for analyzing/inspecting the mixed liquid; and fine fluid circuit (having the width of, for example, a few hundred μm), appropriately connecting these sections to each other. Typically, a microchip is used mounted on an apparatus that can impart centrifugal force to the chip (centrifuge). By applying centrifugal force in an appropriate direction to the microchip, specimen and liquid reagent are measured and mixed, and the mixed liquid can be introduced to the detecting section. The mixed liquid introduced to the detecting section can be inspected and analyzed (for example, a specific component in the mixed liquid can be detected) by optical examination, for example, by irradiating the detecting section containing the mixed liquid with detecting light and examining transmittance thereof (see Japanese Patent Laying-Open Nos. 2006-300741 and 2007-298474).
Here, microchips can roughly be divided, from their shapes, into two types. One is the type (in the following, also referred to as a first type) in which the fluid circuit is formed by joining a substrate having a trench on one surface with another substrate. The other is the type (in the following, also referred to as a second type) in which fluid circuit is formed by joining a substrate having trenches on opposite surfaces with separate substrates on the opposite surfaces.
Microchips of both former and latter types have an air vent to allow smooth movement of liquid through the fine fluid circuit. The air vent connects the fluid circuit to the outside and, in designing a microchip, it must be provided at such a position where leakage of liquid from the air vent can be prevented.
FIG. 26 is a schematic perspective view showing an example of the measuring section and an excess storage of a conventional microchip. FIG. 27 is a schematic perspective view showing another example of the measuring section and an excess storage of a conventional microchip. Microchips shown in FIGS. 26 and 27 are of the first type.
In the following, description will be given with reference to FIGS. 26 and 27. First, the microchip shown in FIG. 26 will be described. The microchip includes a measuring section 80 and an excess storage 74, with an air vent provided at a position 72. In the microchip, a specimen, liquid reagent or mixture (hereinafter, these may simply be referred to as “fluid”) is introduced from a flow path 71 by the application of centrifugal force, and a prescribed amount thereof is measured at measuring section 80. The fluid overflowed from measuring section 80 flows as excess fluid, to excess storage 74. Centrifugal force is first applied in the left direction of FIG. 26, and thereafter, centrifugal force is applied in the upward direction of FIG. 26, whereby the fluid in measuring section 80 moves upward in FIG. 26, and the excess fluid is reserved in excess storage 74. In order to realize such a series of fluid operations, an air vent is provided near the position 72. The vent is positioned not directly above the fluid in operation, but at a position connected to the fluid circuit. In other words, it is necessary to enclose the excess fluid in a maze-like excess storage 74 so that the excess fluid would not flow out from the air vent even when centrifugal force is applied in four directions, for example, and to provide the air vent separately at a portion where contact with the fluid is impossible. The structure shown in FIG. 26 may hinder reduction in size of the microchip.
Next, the microchip shown in FIG. 27 will be described. The microchip includes a measuring section 90 and an excess storage 84, with an air vent provided at position 82. In the microchip, the fluid is introduced from a flow path 81 by the application of centrifugal force, and a prescribed amount thereof is measured at measuring section 90. The fluid overflowed from measuring section 90 flows as excess fluid, to excess storage 84. Next, centrifugal force is applied in the left direction of FIG. 27, so that the fluid in measuring section 90 moves to a next, functioning section, while the excess fluid moves to the left side of excess storage 84 of FIG. 27. Next, centrifugal force is applied in the upward direction of FIG. 27, whereby excess fluid moves upward in FIG. 27 and the excess fluid is reserved in excess storage 84. In order to realize such a series of fluid operations, an air vent is provided near the position 82. The vent is positioned not directly above the fluid in operation, but at a position connected to the fluid circuit. In other words, however, the air vent must be provided near the center of excess storage 84 so that the fluid would not pass directly above the air vent even if centrifugal force is applied in four directions. Therefore, around the air vent, so-called dead spaces result, for holding the excess fluid when centrifugal force is applied in respective four directions. Therefore, the structure shown in FIG. 27 possibly hinders reduction in size of the microchip.
A structure that can make smaller the dead space for excess fluid or the like has been developed also for the microchips of the second type.
In a microchip, an excess storage is provided for containing excess fluid such as specimen or liquid reagent that is determined to be excessive in measuring of specimen and liquid reagent and hence unnecessary for examination. In order not to affect the examination above after once contained in the excess storage, the excess fluid must be kept in the excess storage. Therefore, a microchip having an excess storage occupying a certain area has been disclosed (for example, see U.S. Pat. No. 4,883,763).
FIG. 28 is a schematic perspective view showing an example of a conventional excess storage of a microchip. FIG. 29 is a schematic plan view showing an example of a conventional excess storage of a microchip. The dimensional relation of length, size and width in the figures are appropriately changed for simplicity and clarification and does not represent actual size.
In the following, the structure and operation of conventional excess storage will be described with reference to FIGS. 28 and 29. The microchip shown in FIG. 28 is formed by joining a first substrate 251 and a second substrate 252, and a trench is formed on a surface of first substrate 251. The trench and that surface of second substrate 252 which faces the first substrate 251 form the fluid circuit. The fluid circuit has a flow path 253 and an excess storage 255 coupled to flow path 253. Specifically, by the trench formed on the surface of first substrate 251 and the second substrate 252, flow path 253 and excess storage 255 are formed. The microchip may or may not have an air vent 256 formed therein.
In the microchip shown in FIGS. 28 and 29, excess fluid as the fluid in flow path 253 is moved by applying centrifugal force in the direction indicated by an arrow 263, next in the direction of arrow 262, next in the direction of arrow 261, and thereafter in the direction of arrow 264, thereby the excess fluid can be moved to and contained in excess storage 255. Thereafter, unless the centrifugal force is applied in the direction of arrows 261, 262, 263 and 264 in this order during the operation of the microchip, the excess fluid will not flow back, and the excess fluid is kept contained in excess storage 255.
In other words, however, if a conventional excess storage of such a structure is used, the excess fluid contained in excess storage 255 would flow back unless the order of applying centrifugal force is regulated during the operation of microchip. Further, excess storage 255 having an eddy shape when viewed two-dimensionally such as shown in FIG. 29 occupies a considerable area of the microchip, limiting reduction in size of the microchip.